[Cl-].[Al+3].[Cl-].[Cl-].[Br:5][C:6]1[CH:10]=[C:9]([CH3:11])[S:8][C:7]=1[CH3:12].Cl[C:14](=[O:20])[C:15]([O:17][CH2:18][CH3:19])=[O:16].O>ClCCl>[Br:5][C:6]1[C:10]([C:14](=[O:20])[C:15]([O:17][CH2:18][CH3:19])=[O:16])=[C:9]([CH3:11])[S:8][C:7]=1[CH3:12]
Under a nitrogen atmosphere, aluminium chloride (1.46 g, 11 mmol) was added per portions at 0° C. to a solution of 3-bromo-2,5-dimethylthiophene (17b) (1.05 g, 5.49 mmol) and ethyl chlorooxoacetate (614 μL, 5.5 mmol) in dichloromethane (12 mL). After 2 hours of vigorous stirring, the reaction mixture was slowly hydrolyzed at 0° C. with water. The organic layer was separated, washed with hydrochloric acid aqueous solution 1N, brine, dried over sodium sulfate, filtered and evaporated under reduced pressure. The residue was purified by flash chromatography on silica gel (cyclohexane/ethyl acetate 95/5)) to afford the desired keto-ester (17c) as a orange solid (630 mg, 2.16 mmol, 39%).
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